1. Field of the Invention
This invention relates generally to methods of synthesizing nitrophenols, and more particularly relates to a method of selectively synthesizing 4,6-dinitro resorcinol.
2. Description of the Prior Art
Direct nitration of phenolic compounds is typically highly exothermic and difficult to control. Because nitrophenols are synthetically important compounds, however, their controlled preparation in high yield is desirable. Further, it is also desirable that the number and complexity of intermediate steps be kept to a minimum in order to reduce the time and cost involved in synthesis.
The present invention is directed to the nitration of resorcinol (1,3-dihydroxybenzene; Formula 1). In particular, this invention is directed to a method of selectively synthesizing the 4,6-dinitroresorcinol isomer (Formula 2), which compound is a useful intermediate in the preparation of poly[p-phenylenebenzobisoxazole] ("PBO") and related compounds. As is well known, PBO (Formula 3) is a high-strength, liquid crystalline material with many applications. ##STR1##
Several syntheses of nitrophenols and of nitroresorcinol monomers in particular have been proposed.
U.S. Pat. No. 1,380,186 to Brewster discloses a method of synthesizing picric acid (2,4,6-trinitrophenol) from phenol using a nitric acid based solution. Brewster, however, does not disclose a method that will preferentially produce either mono- or dinitro compounds in substantial quantities.
U.S. Pat. No. 2,945,890 to Allan shows a method of preparing 2,4-dinitroresorcinol using a sulfuric acid/nitric acid nitrating solution. Apparently, however, styphnic acid (2,4,6-trinitroresorcinol), an undesirable and explosive side product, is also produced in fairly substantial quantities.
U.S. Pat. No. 3,933,926 to Salter discloses an alternative method of preparing 2,4-dinitroresorcinol. Salter proposes nitrosation of resorcinol followed by oxidation of the nitroso groups to nitro substituents. Again, styphnic acid is produced as a side product.
Neither of these two latter processes have been modified successfully to produce 4,6-dinitroresorcinol, the isomer to which the present method is directed. One method which has been used to synthesize 4,6-dinitro resorcinol is admixture of resorcinol diacetate with a 1:1 solution of 70% HNO.sub.3 and 90% red fuming HNO.sub.3 ("red" here indicating the presence of NO.sub.2 in solution). This process has been described in G. C. Berry et al., Air Force Technical Report AFML-TR-79-4115 (August 1979). This procedure, however, gives both a low yield of 4,6-dinitroresorcinol--less than 45%--and simultaneous production of styphnic acid in yields ranging from 30% to 60%.
For additional material on nitration of aromatic compounds, reference may be had to K. Schofield, Aromatic Nitration (Cambridge: Cambridge University Press, 1980).
Thus, there is a need in the art for a method of preparing 4,6-dinitroresorcinol in a high yield with a minimum of undesirable side products.